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© Copyright 2020 Qiyu © Copyright 2020 Qiyu Liu Integrated Acousto-optic Devices based on Brillouin Optomechanics Qiyu Liu A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2020 Reading Committee: Mo Li, Chair Kai-Mei Fu Arka Majumdar Program Authorized to Offer Degree: Electrical Engineering University of Washington Abstract Integrated Acousto-optic Devices based on Brillouin Optomechanics Qiyu Liu Chair of the Supervisory Committee: Professor Mo Li Electrical and Computer Engineering Brillouin scattering is the inelastic light-sound interaction. It has given birth to two popular research topics: stimulated Brillouin scattering and acousto-optics. With recent advances in nanotechnology, research on both stimulated Brillouin scattering and acousto-optics has focused on scaling down the traditional bulky material to nanophotonic chips for functional devices. The integrated platforms have not only the advantage of a small footprint but also the flexibility of design and fabrication. On-chip stimulated Brillouin scattering has been demonstrated in varieties of platforms for integrated photonics. The key factor of stimulated Brillouin scattering is the Brillouin gain, which is determined by the intensity of the coherent acoustic waves. Because the acoustic wave frequency is about five orders of magnitude lower than that of light, the energy efficiency of acoustic generation through intense pump light is relatively low. On the contrary, in acousto-optics, acoustic waves are generated piezoelectrically with microwave at the same frequency, which is much efficient. However, previous work in acousto-optics focused on either acoustic modulation of cavities or small deflection of light. The interaction of light and sound with the wavelength in the same scale remains unexplored. My first project focused on exploring new phenomena and inventing novel devices by combining the advantages of stimulated Brillouin scattering and the acousto-optic modulation. With successfully integrating sub-optical wavelength acoustic transducers onto aluminum nitride photonic integrated circuits, we experimentally observed the backscattering of photons with piezoelectrically excited phonons for the first time. We systematically studied the electromechanical Brillouin scattering system. Potential applications as single-sideband modulators and microwave photonic links have also been demonstrated. In the second project, a subwavelength suspended phononic waveguide was realized. The coupling of an acoustic wave was achieved through a parabolic acoustic coupler. Different acoustic couplers with interdigital transducers with different apertures are fabricated for comparison and optimization. Transmission of the Lamb wave at 3.4 GHz was detected even in the presence of high waveguide loss. Furthermore, the phenomenon of mode conversion from membrane acoustic waves to subwavelength guided modes was first visualized through microwave impedance microscopy. This project provides guidance for the future design of co-confining light and sound in subwavelength structures. Besides these two projects, I will also discuss the fundamental of surface acoustic waves and the technical details in surface acoustic wave device design in Chapter 2. Future perspectives for integrated acousto-optic devices are included in the last chapter. i TABLE OF CONTENTS List of Figures ................................................................................................................................ iv List of Tables ................................................................................................................................. vi Chapter 1. Introduction ................................................................................................................... 9 1.1 A brief review of photonic integrated circuits ................................................................ 9 1.1.1 From fiber optics to photonic integrated circuits ........................................................ 9 1.1.2 Recent progress of PIC ............................................................................................. 10 1.2 A brief review of surface acoustic wave ....................................................................... 15 1.2.1 Introduction to surface acoustic wave ....................................................................... 15 1.2.2 Applications of surface acoustic wave devices ......................................................... 16 1.3 Recent research on surface acoustic waves .................................................................. 17 1.4 A brief review of acousto-optics and Brillouin scattering ............................................ 18 1.4.1 Principle of the Brillouin scattering .......................................................................... 18 1.4.2 The exploration of Brillouin scattering ..................................................................... 19 1.4.3 Development of stimulated Brillouin scattering ....................................................... 21 1.5 Content of the dissertation ............................................................................................ 25 Chapter 2. Fundamental of Surface Acoustic Waves ................................................................... 26 2.1 Linear elasticity ............................................................................................................. 26 2.2 The Rayleigh and Lamb wave ...................................................................................... 28 2.3 Piezoelectricity .............................................................................................................. 30 2.4 Photoelasticity ............................................................................................................... 31 i ii 2.5 Surface acoustic wave transducer ................................................................................. 32 2.6 The coupling of mode analysis ..................................................................................... 32 2.7 Surface acoustic wave devices on aluminum nitride membrane .................................. 36 2.8 Guided modes in the optomechanical waveguide ......................................................... 41 Chapter 3. Electromechanical Brillouin Scattering in the Optomechanical Waveguide .............. 42 3.1 Motivations and feasibility ........................................................................................... 42 3.2 Theory of electromechanical Brillouin scattering ......................................................... 45 3.3 Design and Calibration of the optical layer .................................................................. 50 3.4 Design of the acoustic layer .......................................................................................... 53 3.5 Device Fabrication ........................................................................................................ 57 3.6 Demonstration of the electromechanical Brillouin scattering ...................................... 60 3.7 Applications in the RF-photonic links .......................................................................... 67 3.8 Conclusion of the project .............................................................................................. 70 Chapter 4. Subwavelength-scale Phononic Waveguide for GHz-range Lamb Modes ................. 75 4.1 Introduction ................................................................................................................... 75 4.2 Motivation ..................................................................................................................... 76 4.3 Device design ................................................................................................................ 77 4.4 Device Fabrication ........................................................................................................ 79 4.5 Calibration of the surface acoustic wave transducers ................................................... 80 4.6 Estimation of acoustic transmission from S21 measurement ....................................... 84 4.7 Visualization of the surface acoustic wave With the microwave impedance microscopy 89 ii iii 4.8 Simulation of acoustic coupling near the entrance of the waveguide ........................... 91 4.9 Visualization of Lamb wave properties in the coupling region .................................... 91 4.10 Conclusion and outlook ................................................................................................ 93 Chapter 5. Conclusions and Outlook ............................................................................................ 97 5.1 Summary ....................................................................................................................... 97 5.2 Future perspectives ....................................................................................................... 98 iii iv LIST OF FIGURES Figure 1.1 Historical Review of Fiber Optics and Photonic Integrated Circuits.…………….…... 2 Figure 1.2 Principle of the Index Guiding of Photonic Waveguides and Applications of Photonic Integrated Circuits on Telecommunication……………………………….…....………4 Figure 1.3 Recent Research Progress on Novel Functional Devices based on Photonic Integrated Circuit…………………………………………………………….…………………….6 Figure 1.4 The Fundamental of Surface Acoustic Wave Devices….………….….……………….8
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